This invention relates to composite superconductors and more particularly to Bi-2223 superconductors surrounded by other superconducting materials.
The powder-in-tube (PIT) process, which yields a highly textured (Bi, Pb)2Sr2Ca2Cu3Oy (Bi-2223) superconductor with its c-axis aligned parallel to the tape surface, is an industrially scalable technique for fabricating long-length superconductors. In zero applied magnetic field, the critical current density (Jc) of superconducting tape is controlled by its microstructure. Grain boundaries act as barriers to the transfer of transport current between grains. The crystallographic anisotropy of Bi-2223, which exhibits a micaceous or platelike morphology, allows large contact areas alignment of grains with their c-axis perpendicular to the rolling direction of the tape, easy transfer of current across its grain boundaries, and high Jc values. The fabrication of YBa2Cu3O7-xcex4 (Y-123) wires by the PIT technique has not been successful because of YBCO""s granularity. Y-123 exhibits greater isotropy than Bi-2223 and its intergranular transport current is poor because of weak links. In an applied magnetic field, Y-123 shows much better Jc response than Bi-2223 and the irreversibility line (IRL) of high-Tc materials can provide insight into materials limitations for various applications.
Recent progress in growing single crystals of Bi-2223 provided an opportunity to study its IRL line. A sharp irreversibility field (H*) drop was observed between 20 and 40 K. At 75 K, H* was only ≈0.3 T. This observation clearly shows very weak intrinsic pinning in Bi-2223 single crystals. Intrinsic pinning in Y-123 single crystal is sufficiently strong to keep H* at 77 K to ≈8 T. Using ion-beam-assisted deposition (IBAD), c-axis-oriented Y-123 thin films have been deposited on polycrystalline metallic tapes buffered with yttria stabilized zirconia (YSZ). A significant improvement of the in-plane texture led to a high Jc above 106 (A/cm2).
Another method of depositing thin films is called rolling-assisted biaxial textured substrates (RABiT). A biaxially textured nickel substrate was aligned to within a few degrees.
Epitaxial growth of oxide buffer layers on the substrate made possible the growth of Y-123 films that were 1.5 xcexcm thick and exhibited superconducting properties comparable to those observed for epitaxial films on single-crystal substrates. The transport current density as a function of temperature and applied magnetic field of Bi-2223/Ag wires and tapes, show that practical applications are limited to either low temperature and high magnetic field ≈20 K and ≈10 T or high temperature and low magnetic field ≈77K and less than 0.5 T.
It is a general object of the invention to combine the good alignment of Bi-2223 grains in Ag-sheathed superconducting tapes to obtain high Jc values at high temperature and low field, and the good intrinsic pinning of Y-123 thin film in order to maintain high Jc values in high fields.
Another object of the present invention is to provide a new composite tape in which the primary function of a central Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) filaments was to conduct transport current, with a YBa2Cu3O7-xcex4 (Y-123) thin films deposited on the Ag-sheathed Bi2223 tape to shield the applied magnetic field and protect the central Bi-2223 filaments.
Yet another object of the invention was to obtain critical current densities of the Y-123-coated, Ag sheathed Bi-2223 tapes which were better than those of an uncoated tape.
Still other aspects of the invention was that the Y-123 thin film exhibited a Tc≈72K and a broad transition region that shifted the effect to lower temperatures. Pole figure measurements showed widely spread a,b planes along the rolling direction, indicating high-angle grain boundaries that diminished the magnitude of the effect. Microstructural observations showed platelike grains of Y-123 with fine growth ledges in the thin film that was heat treated, in contrast with the microstructure of an as-coated thin film that showed large twinned grains. The experimental results showed that heat treating Y-123 thin film according to the previously known Bi-2223 tape schedule was compatible with an beneficial for Y-123.